Effects Of Sc And Zr Microalloying Additions On The Microstructure And Properties Of A New Type Of Al-Zn-Mg-Sc-Zr Alloys

Al-Zn-Mg alloys have been extensively used for structural applications in the automobile, construction and aerospace industries due to their light weight, excellent mechanical properties and good processability. Rare earth elements additives can further improve the properties of alloys. Adding scandium into aluminum can result in grain refinement, prevention of recrystallization and enhanced dispersoid strengthening. However, the high price of Sc limits its wide application in aluminum alloys. Zirconium, as another efficient and common alloying element with relatively low price, has been proved to decrease the average grain size and to increase tensile strength as well. Thus, it has important values theoretically and practically to study the combined effects of Sc and Zr to obtain a new type of aluminum alloys with high performance and low price.In this work, we designed a new type of Al-Zn-Mg-Sc-Zr alloys with different Sc and Zr additions. Through the microstructure analysis and performance tests on the alloys in different states of casting, homogenization annealing, rolling and heat treatment, the effects of Sc/Zr ratio and the total amount of Sc and Zr on the the microstructure and mechanical properties were investigated. Furthermore, the mechanism of the influence of Sc and Zr elements on the alloys were also investigated.The results showed that both Sc and Zr worked to refine grains. Sc addition into the Al-Zn-Mg alloys refined the grain size efficiently and increase recrystallization resistance, especially together with Zr addition. There was a synergistic enhancement effect on the mechanical properties of the alloys when adding Sc and Zr together. The strength of the alloys increased with the increasing of Sc+Zr content. Compared with the mechanical properties of the Al-Zn-Mg alloy without Sc and Zr aged at 120? for 24 h, the ultimate tensile strength and the yield strength increased by 16.2% and 22.2% after adding 0.2 wt.% Sc and 0.4 wt.% Zr respectively.In the Al-Zn-Mg-Sc-Zr alloys,the refining strengthening effects were derived from the primary Al3(Sc, Zr) particles. The secondary Al3(Sc,Zr) precipitates inhibited the recrystallization behavior of deforming alloy effectively. The effects of precipitation strengthening were derived from the GP zones, ?' phases and secondary Al3(Sc, Zr) precipitates dissolving out of matrix during the aging treatment.